Photoelectron spectra of (CO 2 ) - n have been measured at a photon energy of 4.66 eV, and the vertical detachment energy (VDE) determined as a function of the cluster size, 2 n 16. The n-dependence of the VDE shows sharp discontinuities between n = 6 and 7, and between n = 13 and 14, indicating that CO - 2 and C 2 O - 4 are formed as the core anions in (CO 2 ) - 7 and (CO 2 ) - 1 4 , respectively. From these findings we infer that the anionic core of (CO 2 ) - n changes promptly from CO - 2 to C 2 O - 4 and vice versa when core switching results from the formation of geometrically favored structures.